1. Field of the Invention
Xanthan gum is a heteropolysaccharide produced as a fermentation product by Xanthomonas campestris, a microorganism causing vascular disease of cabbages, cauliflower, and rutabagas. Its structure consists of a linear backbone of .beta.-(1.fwdarw.4)-linked D-glucose residues (linked as in cellulose), which has three-unit-long side chains appended on alternate residues. D-Mannose residues directly appended to the backbone bear O-acetyl substituents on the C-6 position. Pyruvic acetal, i.e., 4,6-0-(1-carboxyethylidene), substituents are on the terminal D-mannosyl residues of some of these side chains, their frequency of occurrence depending on the bacterial strain and fermentation conditions.
Xanthan gum has considerable industrial significance and a variety of applications as summarized by A. Jeanes in "Applications of Extracellular Microbial Polysaccharide-Polyelectrolytes: Review of Literature, Including Patents," J. Polym. Sci., Polym. Symp. No. 45, pp. 216-221 (1974). The stability of its reheological properties under diverse chemical conditions accounts in part for its versatility. For example, the gum is an effective brine thickener for use in drilling mud compositions and also in the secondary and tertiary recovery of petroleum. High-viscosity solutions are employed to carry proppage to fractured rock formations in oil bearing strata. Viscosity is then reduced and the proppage remains to allow better passage of natural gas and petroleum in underground formations in which the temperature often exceeds 50.degree. C. It is essential that an enzymic viscosity breaker for xanthan gum-based hydraulic fluids function under conditions of elevated temperature and salinity. A detailed description of its role in oil recovery is given by Wernau in U.S. Pat. No. 4,119,546.
2. Description of the Prior Art
While the literature is replete with reports on the production, characterization, properties, and applications of xanthan gum, there is a paucity of information on its biological degradation. M. Rinaudo et al. [Chem. Abstr. 92: 176420a (1980)] investigated the mechanism of enzymic hydrolysis by a cellulase. In salt-free solution, a random breakdown of the main chain was observed when the polysaccharide was in the unordered conformation. However, there was no hydrolysis of the more commonly occurring, ordered or helical conformation.
Cripps et al. [Eur. Pat. Appl. 30,393; Chem. Abstr. 95(258): 146157q (1981)] described isolation of a Corynebacterium sp. (NCIB 11535) from a soil enrichment culture on xanthan gum as sole carbon source. The extracellular xanthanase produced aerobically by the organism during growth in the presence of xanthan gum also depolymerized carboxymethyl cellulose. At least nine reaction products were detected by thin-layer chromatography after native xanthan gum in distilled water was incubated with the Cripps enzyme at 30.degree. C. Only four products were obtained from the deacetylated polysaccharide, and these were characterized. In addition to D-mannose and its pyruvic acetal, two were shown by compositional and methylation analyses, and by analogy with the known structure of xanthan gum, to be linear oligosaccharides shown below: EQU 4-ene-GlcA.beta.1-2Mana-3Glc.beta.1-4Glc EQU Man.beta.1-4GlcA.beta.1-2mana1-4Glc
Light absorbance at 232 nm and a positive thiobarbituric acid test suggested that the tetrasaccharide was terminated by an unsaturated (4-ene-) glucuronic acid residue formed through action of a lyase. Presence of two glucose residues in both of the linear oligosaccharides indicated that the hydrolase component of the enzyme complex attacked .beta.-(1.fwdarw.4)-glucosyl linkages to the glucosyl residues bearing side chains.
Although the corynebacterial xanthanase of Cripps showed activity at 70.degree. C., it did not have good thermal stability; 30% of the activity was lost in 1 hr at 30.degree. C.
Salt-tolerant bacteria that produce a xanthanase complex functional in the presence of brines have been isolated on enrichment culture with xanthan gum as the main carbon source [M. C. Cadmus et. al., "Biodegradation of Xanthan Gum by Bacillus sp., " Appl. Environ. Microbiol. 44:5 (1982); and U.S. Pat. No. 4,410,625]. It is further noted that the enzyme complex displayed resistance to thermal inactivation at 48.degree. C. in the presence of 4-10% NaCl [M. C. Cadmus et al., "Enzymic Breakage of Xanthan Gum Solution Viscosity," Dev. Ind. Microbiol. 26:281 (1985)]. In many rock formations, however, the temperature exceeds 60.degree. C., and a more heat-stable enzyme is required.